The invention is disclosed in the following in the context of MR (magnetic resonance) imaging. However, it has to be noted that the invention is in the same manner applicable in other fields in which arrays of antennas are used for sensitive reception of RF signals, such as, e.g., in the fields of radio communication technology, mobile communication, radar, or radio astronomy.
Image-forming MR methods, which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images, are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive.
According to the MR method in general, the body of the patient to be examined is arranged in a strong, uniform magnetic field whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency (so-called Larmor frequency, or MR frequency).
From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field extends perpendicular to the z-axis, so that the magnetization performs a precessional motion about the z-axis.
After termination of the RF pulse, the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant T1 (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time). The variation of the magnetization can be detected by means of receiving RF antennas (coils) which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis.
In order to realize spatial resolution in the body, linear magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving antennas then contains components of different frequencies which can be associated with different locations in the body. The signal data obtained via the receiving coils is converted to an MR image, e.g., by means of Fourier transformation.
It is well known in the art (see, e.g., U.S. Pat. No. 4,947,121) that by using arrays of two or more receiving antennas for receiving MR signals, multiple datasets can be simultaneously acquired and that these datasets can be combined for significantly improving the sensitivity, i.e. the signal-to-noise ratio (SNR), and/or the field-of-view (FOV). Such arrays consist of closely positioned receiving antennas (e.g. surface coils) which receive MR signals from a common region of interest within the examination volume of the respective MR apparatus. Each receiving antenna is connected, via a matching network, to a low-noise amplifier. Each chain consisting of receiving antenna, matching network and low-noise amplifier constitutes a part of a receiving channel of the MR apparatus. Each receiving channel may include further components, such as an analog-to-digital converter, switches et cetera. The outputs of the channels, each comprising signal and noise, are combined during the image reconstruction process. The combined SNR is influenced by the performance of each individual antenna, the noise correlation between the antennas, as well as by the noise parameters of the low-noise amplifiers.
In contemporary MR systems noise coupling between the individual adjacently positioned receiving antennas is known to have a limiting effect defining the maximum number of antennas within the array to obtain moderate SNR. Modern MR scanners, however, sometimes include arrays with a plurality of receiving antennas by far exceeding this maximum number. This is because recently parallel acquisition techniques have been developed. Methods in this category are SENSE (Pruessmann et al., “SENSE: Sensitivity Encoding for Fast MRI”, Magnetic Resonance in Medicine 1999, 42 (5), 1952-1962) and SMASH (Sodickson et al., “Simultaneous acquisition of spatial harmonics (SMASH): Fast imaging with radiofrequency coil arrays”, Magnetic Resonance in Medicine 1997, 38, 591-603). SENSE and SMASH use undersampled k-space data acquisition obtained from multiple RF receiving antennas in parallel. In these methods, the (complex) signal data from the multiple coils are combined with complex weightings in such a way as to suppress undersampling artifacts (aliasing) in the finally reconstructed MR images. In either SENSE or SMASH, the acceleration of image acquisition grows with the number of receiving antennas used. Thus a maximum number of receiving channels is generally desirable in order to increase imaging speed as much as possible.
Usually in today's MR systems the individual receiving antennas are decoupled by overlap. Since more distant antennas of the array can not be decoupled in this way, so-called pre-amplifier decoupling is used as an additional measure. This approach makes use of the fact that the input impedance of each low-noise amplifier differs from the optimum impedance that provides an optimal noise performance, i.e. a maximum SNR, of the respective channel. The matching network of each channel, which is switched between the antenna and the low-noise amplifier, transforms the optimum impedance of the respective low-noise amplifier. This type of matching still has a degree of freedom which is used to maximize the impedance as presented to the antenna. In this way the current within each antenna is reduced. However, residual coupling between the antennas is still remaining in the known systems which leads to noise coupling from one low-noise amplifier to the others, thereby reducing the total SNR.
From the foregoing it is readily appreciated that there is a need for an improved method for optimizing the signal-to-noise ratio in a system comprising an array of coupled RF receiving antennas.